1. Field of the Invention
The present invention relates to a technology for superimposing a plurality of different single-color images to obtain a color image.
2. Description of the Related Art
An image forming apparatus, in which a latent image is written on a photoconductor serving as an image carrier by an optical beam such as a laser beam, visualized by a developing device, and transferred onto a recording medium such as transfer paper, is widely used for a copier, a printer, a facsimile machine, a multifunction product, and the like. A color image forming apparatus capable of color image processing is in widespread use in response to growing market demand. As the color image forming apparatus, a so-called tandem type color image forming apparatus is widely used because high-speed image forming can be easily achieved. In the tandem type color image forming apparatus, a plurality of photoconductors each including a developing device are arranged in parallel, and single-color toner images formed on the respective photoconductors are sequentially transferred onto transfer paper to form a full-color image thereon.
In the color image forming apparatus, at the time of superimposing colors, color unevenness sometimes occurs in an image due to deviation of a color-superimposing position from a target position. Such a deviation is caused by, for example, rotational fluctuation generated cyclically for each one rotation of the photoconductors. The rotational fluctuation is corrected by a method in which a rotation detector including four slits is arranged on a shaft of the photoconductor, and a reference clock cycle of a motor serving as a rotation drive source for the photoconductor is adjusted to remove a fluctuation component. However, the rotational fluctuation of the photoconductor cannot be sufficiently removed by the method, thereby causing out-of-color registration.
For example, Japanese Patent Application Laid-Open No. 2002-268315 discloses an imaging device, in which one encoder is coupled with a plurality of photoconductors to detect fluctuations in one rotation of the respective photoconductors to adjust phases of the rotational fluctuation. Japanese Patent Application Laid-Open No. H9-127755 discloses a color image forming device, in which an encoder is attached to a photoconductor for a reference color, and rotational fluctuation of a photoconductor motor is detected based on an output signal from the encoder and deviation between resist marks transferred onto a transfer member as toner images.
In the imaging device, however, because of the belt-coupling, it is difficult to accurately detect fluctuations in one rotation cycle by the encoder due to environmental changes or belt characteristics. In addition, the respective photoconductors are coupled with one encoder, and therefore, the mechanism is complicated and large.
In the color image forming device, rotational fluctuation of the transfer member appears in between the resist marks of toner images. Therefore, it is difficult to accurately detect fluctuation of the photoconductor. When belt transfer is used, the fluctuation is also affected by belt expansion and contraction.